Mohammed Haydar Zammar, a Syrian-born German national, is accused of being involved in planning the 9/11 attacks.

A Kurdish military commander based in Syria's north has announced that his forces arrested Mohammed Haydar Zammar, a Syrian-born German national accused of being involved in the planning of the September 11, attacks in 2001.

"Mohammed Haydar Zammar has been arrested by Kurdish security forces and is now being interrogated in northern Syria," the Kurdish Peoples' Protection Units (YPG) commander, who wishes to remain unnamed, said on Thursday.

He was previously questioned by German police following the September 11 attacks, in which at least 3,000 people were killed.

Two planes deliberately crashed into the twin towers of the World Trade Center in New York City, and a third hit the Pentagon in Washington, DC. The fourth plane crashed into a field in Pennsylvania.

Zammar, who has been accused of recruiting some of the plane hijackers, left for Morocco later in 2011 before being arrested in the North African country as part of an operation involving CIA agents. He was then handed over to the Syrian authorities.

US authorities suspect that Zammar recruited Mohammed Atta, an Egyptian national who was one of the pilots involved in the 9/11 attacks, since both used to reside in Germany's Hamburg.

The Washington Post had quoted US sources as saying that Syrian authorities had arrested Zammar during the early years of the Syrian war, now in its eighth year, but he was later released in 2014.

The Pentagon said it was looking into Zammar's capture, but had nothing to confirm at this time.